The invention relates to a measuring apparatus for measuring body parts and other workpieces, in which a sensor, which is fastened to a movable arm of an industrial robot, checks the dimensional accuracy of the workpieces. In particular, the invention relates to the replacement of a calibration body which is measured by the sensor for calibration purposes.
Measuring cells, in which the processed workpieces are checked in terms of the dimensional accuracy thereof or in terms of other manufacturing parameters, are more and more often integrated into modern assembly lines in the automotive industry and in other branches. Since the check is carried out during an ongoing manufacturing process, the workpieces in the measuring cells need to be measured not only with a high accuracy (typically of the order of 100 μm) but also very quickly. Moreover, the measuring cells must operate very reliably since failures of the measuring apparatuses may severely impair the operation of the assembly line.
In order to satisfy these demands, known measuring apparatuses of this type, as a rule, comprise an industrial robot having a movable arm. A sensor with which it is possible to measure the pose of structures of the workpieces relative to the sensor is fastened to the movable arm. By way of example, these structures may be bores, edges, folds or seams. An industrial robot is understood to mean a universally employable automatic movement unit with a plurality of axes, the movements of which are freely programmable and, optionally, sensor-guided in respect of the movement sequence, the movement paths and the movement angles. The combination of position and orientation of a body in the Cartesian space is referred to as “pose”. Three Cartesian coordinates and three angle coordinates are usually used to specify the pose. As a rule, the sensors fastened to the arm of the industrial robot are optical sensors since the pose of structures of the workpieces may thus be measured quickly, contactlessly and with a high accuracy.
If the measuring device determines intolerably large deviations between the measured values and the intended values, the relevant workpiece is separated out in a subsequent step and optionally post-processed in order to be able to be used again.
As a rule, it is necessary for the workpieces to be measured in respect of an external coordinate system which is stationary in space. Since the sensor is only able to measure the pose of structures of the workpieces relative to the sensor, the position of the sensor must therefore also be exactly determinable in the external coordinate system. Therefore, the industrial robot needs to displace the sensor with a high accuracy to the desired position relative to the workpiece.
A distinction is made between the absolute accuracy and the repetition accuracy in the case of industrial robots. The absolute accuracy is understood to mean the maximum deviation between an expected intended pose and the actual pose, which emerge when approaching the intended pose from different directions. The repetition accuracy specifies how accurately a robot may be positioned in the case of multiple approaches of a pose from the same direction. For the measurement problems considered here, both the absolute accuracy and the repetition accuracy must be very high because the workpieces can only be measured precisely in the external coordinate system in this case. Restrictions in the absolute accuracy may be compensated for by a correlation with an external measurement system and the offsets resulting therefrom.
The repetition accuracy is impaired by thermal influences in particular. On account of the relatively large dimensions of industrial robots, temperature variations of a few degrees centigrade may lead to deviations of the actual pose of the TCP (tool center point; specifies the tool work point and hence the location of the sensor) from the intended pose of the order of one millimeter.
In order to improve the repetition accuracy of industrial robots, these are usually calibrated at regular time intervals, to be precise typically during the ongoing manufacturing operation. In the measuring apparatuses considered here, the calibration is carried out by virtue of the sensor carried by the industrial robot approaching a calibration body which is arranged stationary in space and measuring the pose of a calibration element fastened to the calibration body at that location. The calibration elements are often spheres since these look the same when viewed from any direction. If the pose of the calibration element is exactly known in the external coordinate system, it is possible to calibrate the measuring apparatus by comparing the pose of the calibration element as measured by the sensor to the precisely known pose thereof. Usually, the pose of not only one calibration element, but of a plurality of calibration elements is measured.
Calibration methods of this type are described in EP 1 189 732 B1 (corresponds to U.S. Pat. No. 6,615,112 B1). There, the calibration body is embodied as a calibration table, the calibration body having a plane surface on which measurement marks which, for example, may be embodied as circular thin platelets or circular openings are arranged.
Moreover, the prior art has disclosed calibration bodies having a hollow cylindrical carrier, on the circumferential side of which a plurality of spherical calibration elements are mounted. The carrier is fastened to a base plate or another carrying structure.
A problem arising in practice independently of the embodiment of the calibration body is that the calibration body may be damaged during the operation of the measuring apparatus. This is caused by often incorrectly programmed movements of the industrial robot or workpieces which have inadvertently become detached from a holder. Since even small deformations of the calibration body are immediately reflected in a reduced measurement accuracy, the calibration body must be remeasured in the external coordinate system; this requires much time outlay and interrupts the manufacturing process for a relatively long time.
Therefore, it is often more advantageous to replace the calibration body with a new calibration body, the dimensions of which were determined exactly by a preceding measurement. Then, the new calibration body is aligned on the carrying structure with the aid of an alignment element. The dimensions of the new calibration body, specifically, in particular, the pose of the calibration elements relative to the alignment element which sets the pose of the calibration body relative to the base plate or any other carrying structure, were previously stored on a data medium and supplied to an evaluation unit of the measuring apparatus.
However, such replacement of a calibration body also leads to relatively long outage of the measuring apparatus since the application of the data which specify the pose of the calibration elements relative to the alignment element requires system knowledge and may therefore only be carried out by an employee of the measuring apparatus manufacturer or appropriately educated staff.